This invention relates to processes for forming seals and more particularly to processes for forming glass-to-metal seals in covers for hermetically sealed electrochemical cells.
Modern electrochemical cells employ highly reactive materials such as lithium or other alkali metals. These cells use non-aqueous electrolytes which can be organic or inorganic. An example of a currently contemplated inorganic electrolyte is one comprised of a solvent of thionyl chloride containing a solute of lithiumaluminum tetrachloride. Carbon is often employed as the cathode in such cells.
Because of the high energy available from these reactive materials, cells employing them are readily adaptable to small sizes such as button cells. The term "button cell" is employed herein to mean cells having disc-like configuration with a diameter less than two inches (5.08 cm) and a thickness usually of less than one-half inch (1.27 cm).
Such cells are used in delicate electronic equipment such as watches, cameras, hearing aids and pacemakers where their small size and high capacity are most effective. In this equipment, which is often quite expensive and in contact with the human anatomy, it is extremely important that the cells remain leakproof so that the highly reactive ingredients thereof cause no harm.
In conventional cells of this type a container is employed which is electrically conductive metal and which forms one of the terminals of the cell. A cover of like material is welded to the bottom half of the container and thus assumes the same polarity. The other terminal projects through the cover and is isolated therefrom by means of an electrically insulating material such as rubber, neoprene, glass or ceramic. In those cells employing a glass or ceramic-to-metal seal, an aperture is provided in the cover. This aperture is larger than the diameter of the terminal to project therethrough and is filled with the glass or ceramic material. Obviously, the insulating material should have substantially the same thermal coefficient of expansion as the metal of the container.
The seal area of these cells has presented the most persistent leak problem associated with the cells. The glass seal develops cracks which sooner or later allow the elctrolyte to escape.
It has been determined that at least one cause of cracks in the glass seal is a grinding operation performed after the seal is made. The grinding operation is to insure that the inside surface of the terminal lead (i.e., the surface which will be at the interior of the complete cell) is flushed with the glass seal. Subsequently, an electrically conductive current collector is welded to this surface of the terminal lead.